This invention relates to methods and devices for cooling bodies such as semiconductor substrates after they are heated. More particularly, the invention relates to cooling a substrate by heat transfer between the substrate and a heat sink prior to handling the substrate.
Semiconductor wafers or other such substrates are subjected to very high treatment or processing temperatures. For example, in certain chemical vapor deposition (CVD) processes, the temperatures can approach 1,200° C. In a typical cycle, a wafer is transferred from a room temperature cassette by a robotic wafer handler into a processing or reaction chamber, where it is subjected to a high temperature treatment and is then transferred by the wafer handler from the high temperature processing chamber back to the same cassette or a separate cassette for processed wafers.
In many high temperature processes encountered in semiconductor processing, the wafer has to cool from the processing temperature to a much lower temperature before it can be placed or picked up by a wafer handler. For example, in an epitaxial silicon deposition reactor, processing temperatures in the reaction chamber are typically in the range of 1,000–1,200° C., while the maximum temperature that the robotic wafer handler can handle is only about 900° C. Furthermore, at high temperatures, the wafer is more vulnerable to physical damage which can be caused by the wafer handler during transportation. Therefore, the wafer must be allowed to cool down from the processing temperature (e.g., to about 900° C.) before it can be handled and transferred by standard handling equipment. Similarly, the wafer must be cooled down to even lower temperatures for safe handling by other types of wafer handlers (e.g., paddles), and for storage in low cost cassettes.
The time required to cool down the wafer to handling temperatures can be very costly to the integrated circuit manufacturer. Cool down rates depend in part upon the mass of the system being cooled, and have been measured at about 45 seconds from 1,200° C. to 900° C. for a 200 mm wafer on typical susceptor. This cool down adds to the total cycle time for each wafer and hence decreases the throughput of the system. This will increase the cost of wafer processing.
Because of the high cost of semiconductor wafer processing equipment, it is critically important from a competitive standpoint to be able to keep the expensive equipment in continued use, allowing increased throughput. At the same time, the wafer cooling technique employed must be compatible with the environment of the CVD processing apparatus and stringent purity requirements. Additionally, the cost of the technique must itself be low enough that there is a net reduction in the per-wafer cost of processing.
It is therefore an object of the present invention to provide a method and apparatus to cool down a wafer quickly and in a uniform fashion from a high processing temperature to a temperature at which the wafer can be picked up using a wafer handler and placed in wafer storage cassette.